1. Technical Field
The present invention relates to a turbo vacuum pump suitable for an application in which a relatively large amount of gas is discharged or evacuated, and more particularly to a turbo vacuum pump which has a high discharge rate at a pump suction port pressure in the range of 1 to 1000 Pa.
2. Related Art
FIG. 22 shows a turbo vacuum pump 1C as an example of conventional turbo vacuum pump. Currently, turbo molecular pumps are widely used as turbo vacuum pumps for processing a semiconductor in semiconductor manufacturing apparatuses or the like.
The turbo vacuum pump 1C has a discharge section L including an impeller discharge section L1 and a groove discharge section L2 constituted of a rotor (rotating member) R and a stator (stationary member) S in a cylindrical pump casing 101 extending vertically.
A lower part of the pump casing 101 is surrounded by a pump base part 102 having a discharge port 120 in communication with the discharge side of the groove discharge section L2. The pump casing 101 with a suction port 101a has, at its upper part, a flange (not shown in FIG. 22) connectable to a device or a pipe from which gas is to be discharged. The stator S has a stationary cylindrical part 103 erected at the center of the pump base part 102 and fixed parts of both the impeller discharge section L1 and the groove discharge section L2.
The rotor R has a rotating shaft 104, which is inserted in the stationary cylindrical part 103, and a rotating cylindrical part 105 attached to the rotating shaft 104. The stationary cylindrical part 103 is housed in a hollow part 105a of the rotating cylindrical part 105. A driving motor 106, and an upper radial bearing 107 and a lower radial bearing 108 located above and below the driving motor 106 are disposed in a gap between the rotating shaft 104 and the stationary cylindrical part 103. An axial bearing 111 having a target disk 109 at the lower end of the rotating shaft 104 and upper and lower electromagnets 110a and 110b on the stator S side is located below the rotating shaft 104. This configuration allows the rotor R to rotate at a high speed under five-axis active control.
The rotating cylindrical part 105 has rotating impellers 112 integrally formed on upper and lower outer peripheries thereof to form an impeller. Stationary impeller 113 is formed on the inner surface of the pump casing and arranged alternately with the rotating impellers 112. When the rotating impellers 112 rotate at a high speed, the impeller discharge section L1 discharges gas by a reciprocal action of the rotating impellers 112 which are rotating and the stationary impellers 113 which remain stationary. The stationary impellers 113 are pressed at their peripheries from above and below and fixed with stationary impeller spacers 114.
The groove discharge section L2 is located under the impeller discharge section L1. That is, the stator S has a spiral groove part spacer 119 surrounding the rotor R and having a spiral groove 119a. The groove discharge section L2 discharges gas by a drag effect of the spiral groove 119a facing the rotor R rotating at a high speed (Patent Document 1, for example).
By placing the groove discharge section L2 downstream of the impeller discharge section L1, a wide range turbo vacuum pump 1C which can operate over a wide range of flow rate can be achieved. Although the spiral groove of the groove discharge section L2 is formed on the stator S in this example, the spiral groove can be alternatively formed on the rotor R in some cases.
As described above, a composite type turbo vacuum pump having a combination of a turbine impeller as a rotating impeller which can discharge gas efficiently in a molecular flow region and a rotor with a spiral groove which can discharge gas in an intermediate flow region has become mainstream. Such a composite type turbo vacuum pump is suitable for an application in which a relatively large amount of gas flows.
The conventional turbo vacuum pump, however, has a feature that the gas discharge rate droops with an increase of pump suction pressure in a high-pressure region of 1 Pa or higher. Therefore, a large-size pump is required to cope with a high flow rate and a low pressure.
It is needless to say that the rotating cylindrical member is to be rotated at as high a speed as possible to improve the discharge performance of the turbo vacuum pump. In a general turbo molecular pump, however, the rotating cylindrical member constituting an impeller surrounds a stationary cylindrical member constituting a stator. Thus, the rotational speed of a turbo vacuum pump is limited by the stress which is generated in the maximum inner diameter part of the rotating cylindrical member. Since the conventional turbo vacuum pump has a limitation on its rotational speed, a pump with a high discharge rate, that is, a pump having a large-diameter turbine impeller is required to achieve a relatively high flow rate and a low pressure, resulting in an increase in size of the pump.
Also, since the rotating cylindrical member is formed as described above, the rotating cylindrical member is required to have a structure of a unitary body. Therefore, when a part of the rotating cylindrical member is damaged, deformed or corroded, it is highly possible that the entire rotating cylindrical member needs to be replaced. This is disadvantageous for long-term use.
In view of the above problems, it is an object of the present invention to provide a turbo vacuum pump in which rotating impellers with high discharge efficiency can be rotated at a higher speed in a pressure range of 1 to 1000 Pa to achieve a high flow rate and a low pressure, that is, a high discharge rate without a large-diameter pump impeller and which is advantageous for long-term use.